Electrolysis bath for electrodepositing silver-tin alloys

ABSTRACT

The invention relates to an electrolysis bath for electrodepositing silver-tin alloys that, in addition to water serving as a solvent with a pH value of less than 1.5, contains a water-soluble silver compound, a water-soluble tin compound and an organic complexing agent. In order to obtain a stable electrolysis bath that enables the homogenous deposition of a compact tin-silver alloy with any type of composition, an aliphatic complexing agent having a sulfide group and an amino group is used as a complexing agent, whereby the functional groups are bound to different carbon atoms.

The invention relates to an electrolytic bath for electrodepositing silver-tin alloys.

A first precondition for the electrodeposition of tin-silver alloys is a stable, aqueous electrolyte solution as the electrolytic bath. To this end, it is necessary to prevent unwanted precipitation of, in particular, the solvated silver ions which, in conjunction with many anions, form sparingly soluble precipitates and require the electrolytic bath to be replaced at an unduly early point.

Apart from the permanent stabilization of the solution there are further problems relating to the considerable difference in standard potentials of the two metals. A large difference in deposition potentials causes disadvantageous dendritic deposition in the form of black, moss- or dendritic-like layers having low adhesivity. For compact layers on the electrode material to be obtained whose adhesivity will meet particular requirements it is necessary that tin and silver be deposited simultaneously as far as possible. That, however, requires the deposition potentials to be largely equalized. Such equalization can be achieved, for example, by complexing the silver ions with the aid of complexants.

EP 0 666 342 B1 describes an electrolytic bath which comprises a water-soluble silver compound, a water-soluble tin compound and a complexant, the complexant being a mercaptoalkanecarboxylic acid, a mercaptoalkanesulfonic acids or a salts of these compounds. By complexing the silver with mercaptoalkanoic acids, however, it is not possible for the deposition potentials to be shifted sufficiently closely toward one another, which means the preparation of smooth, semisolid layers is possible only with specific rather than with any alloy combinations.

U.S. Pat. No. 6,099,713 discloses an electrolytic bath for depositing a tin-silver alloy, the bath comprising silver ions, tin ions, aromatic mercapto compounds and aliphatic thiourea. Thiourea has two amino groups and a sulfide group, these functional groups being bound to a carbon atom. When the silver is complexed with thiourea, four-membered chelate rings are formed. The silver-thiourea complex likewise does not, however, allow the deposition potentials of tin and silver to be shifted sufficiently closely toward one another, which means that, again, it is not possible, using this electrolytic bath, to obtain alloy layers having a composition chosen at liberty and the desired quality.

EP 0 854 206 discloses an electrolytic bath comprising tin ions and silver ions, and aromatic thiol or sulfide compounds as complexants for the purpose of complexing the silver ions. The disclosed aromatic complexants have free amino groups as well as aromatic nitrogen atoms. In some of the compounds disclosed therein, the free amino groups are bound to carbon atoms adjacent to carbon atoms linked to a sulfide group. The rigid aromatic compounds or aryl groups of these ligands having at least six carbon atoms do, however, form bulky and voluminous silver complexes which, as their concentration in the electrolytic bath increases during the electrodeposition, produce a blocking film on the metal surface on which the alloy is to be deposited. Metal ions or other ions additionally present in the electrolyte, for example customary additives, are consequently hindered from penetrating to the metal surface. This spatial hindrance in turn causes undesirable dendritic deposits to be formed.

It is an object of the invention to provide a stable electrolytic bath which enables the homogeneous deposition of compact tin-silver alloy in adjustable compositions.

This object is achieved by an electrolytic bath, which in addition to water as a solvent with a pH of less than 1.5 comprises a water-soluble silver compound, a water-soluble tin compound and an organic complexant, wherein the complexant is an aliphatic complexant and has a sulfide group and an amino group which are linked to different carbon atoms.

The invention is based on the insight that multidendrate complexants having an amino group and a sulfide group form at least five-membered stable chelate complexes which, owing to their high stability compared with four-membered chelate rings of the same composition, ensure the desired convergence of the deposition potentials of tin and silver. Surprisingly, it emerged that dispensing entirely with aryl radicals, which often given rise to toxicity, in the molecular structure of the complexant does not merely result in unimpaired electrodeposition of tin-silver alloys. Rather, owing to the more flexible configuration of the complexant, it is possible, in addition, to overcome further drawbacks of the prior art related to blocking of the electrode surface, thus now making it possible to achieve layers having adjustable alloy compositions while essentially maintaining quality. At the same time, the complexant according to the invention is inexpensive to prepare, moreover allowing the use of aromatic substances to be dispensed with during the preparation process.

According to a preferred further development of the invention, the sulfide group and the amino group are linked to adjacent carbon atoms. Thus, with the incorporation of the silver ion as the central atom, five-membered chelate rings are formed which are particularly stable. This embodiment of the invention therefore allows virtually complete equalization of the deposition potentials of tin and silver.

Within the scope of the invention, the formation of six- or higher-membered chelate rings is also possible, however, a further carbon atom or even more bridging carbon atoms being interposed between the carbon atoms each linked to one of the abovementioned functional groups.

It is further advantageous to limit the size of the complexant, thereby further reducing the risk of undesired layer formation on the relevant electrode surface. According to a further variant of the invention, the complexant therefore has fewer than seven carbon atoms.

Sulfide radicals can form sulfide bridges, but these have essentially no effect on complexing the silver for the purpose of the present invention. The invention therefore also includes the use of complexants which have a sulfide bridge or, in other words, are a disulfide compound.

Expediently, the complexant is a compound from the group consisting of cysteamine, cysteine, cystamine or cystine.

Advantageously, the concentration of the silver ions is 0.1-20 g/l, based on atomic silver.

In an expedient further development of the invention, the concentration of the tin ions is between 1-50 g/l, based on atomic tin. For each batch of the electrolytic bath it has to be borne in mind, however, that the total concentration of the metal salts, acids and organic compounds dissolved in the electrolyte, said total concentration depending not only on the concentration of the water-soluble metal compounds but also on that of the necessary admixtures such as acids, additives and the like, must not exceed a predefined maximum or limit value above which a saturation phenomena will give rise to undesirable precipitation.

The material amount of complexant used according to the invention depends on the material amount of the silver to be complexed, the particle amount of the complexant having to be at least equal to the particle amount of the dissolved silver.

Expediently, the complexant is therefore employed in excess, based on the dissolved silver ions, the particle concentration of the complexant therefore being greater than the particle concentration of the silver ions. Advantageously, the ratio between the material amount of complexant and that of the silver ions is 1.5. The amounts of complexant is to this end as a rule varied between 0.1-50 g/l.

Potentially suitable as a tin compound, according to a further variant of the invention, is a compound from the group of Sn(II) halides, Sn(IV) halides, alkali metal stanates, tin alkanesulfonates or is ammonium stanate, Sn(II) sulfate or tin oxide (SnO). The use of tin citrate or tin oxalate is also possible.

The silver compound is advantageously a compound from the group consisting of the silver halides, the silver alkanesulfonates or the silver diamine complexes. Also suitable, of course, are silver nitrate, silver sulfate and silver oxide (Ag₂O) to be used as the water-soluble silver compound.

The adjustment of the pH of the electrolytic bath is expediently effected by means of an alkanesulfonic acid such as e.g. methylsulfonic acid, ethylsulfonic acid, hydroxypropyl sulfonic acid, phenolsulfonic acid or benzyl sulfonic acid. The amounts of alkanesulfonic acid to be used to adjust the pH of less than 1.5 usually vary between 50-550 g/l.

In a preferred refinement of the invention, methylsulfonic acid is used in addition to silver methylsulfonate and tin methylsulfonate.

Of course it is also possible, within the scope of the invention, to add to the electrolytic bath not only the abovementioned components but also known admixtures or additives in customary amounts. Particularly relevant in this context are brighteners, wetting agents and conducting salts. Examples of conducting salts used for this purpose include boric acid, carboxylic acids and hydroxy acids, the carboxylic acids used being, in particular, formic acid, acetic acid, oxalate acid. Examples of potentially suitable hydroxy acids include citric acid, maleic acid, tartaric acid, gluconic acid, glucaric acid or glucuronic acid. Salts or mixtures of the abovementioned compounds can also be used.

Examples of known brighteners include hexamethylene-tetramine, triethanolamine, acetophenone, formaline or the like. Commercially, brighteners are supplied, for example, under the designation Shipley Rhonal SoldrON.

In the electrolytic bath according to the invention, the desired tin-silver alloy deposition can expediently be carried out at current densities of between 1-10 A/dm². The temperature of the electrolytic bath should not exceed 40° C. and is expediently in the room temperature region, in particular at 25° C.

The invention is described below with reference to working examples.

EXAMPLE 1

An electrolytic bath having a pH of less than 1 was prepared using the following components:

26 g/l of tin methylsulfonate corresponding to 10 g/l of Sn²⁺, based on atomic tin;

3.4 g/l of silver methylsulfonate corresponding to 1.8 g/l of Ag⁺, based on atomic silver;

2.0 g/l of cysteamine (2-aminoethanethiol) having the empirical formula C₂H₇NS;

125 ml/l of 98% strength methylsulfonic acid having the empirical formula HSO₃CH₃;

20 ml of commercial wetting agent, in this case SolderON BTD Carrier™ (ShipleyRonal);

20 ml of commercial brightener, in this case SolderON BTD Additive™ (ShipleyRonal);

water

At an electrolytic bath temperature of 25° C. and a current densities of 1 A/dm², semisolid and semibright tin-silver alloy layers having a tin content of 92 wt. % and a silver content of 8 wt. % were obtained.

EXAMPLE 2

An electrolytic bath having a pH of less than 1 is prepared using the following components:

130 g/l of tin methylsulfonate corresponding to 50 g/l of Sn²⁺, based on atomic tin;

6 g/l of silver methylsulfonate corresponding to 3.2 g/l of Ag⁺, based on atomic silver;

5.4 g/l of L-cysteine having the empirical formula C₃H₇NO₂S;

350 ml/l of 98% strength methylsulfonic acid having the empirical formula HSO₃CH₃;

20 ml of commercial wetting agent, in this case SolderON BTD Carrier™ (ShipleyRonal);

20 ml of commercial brightener, in this case SolderON BTD Additive™ (ShipleyRonal);

water

At an electrolytic bath temperature of 25° C. and a current density of 5 A/dm², well-adherent and semi-bright tin-silver alloy layers having a tin content of 97 wt. % and a silver content of 3 wt. % were obtained.

EXAMPLE 3

An electrolytic bath having a pH of less than 1 is prepared using the following components:

65 g/l of tin methylsulfonate corresponding to 25 g/l of Sn²⁺, based on atomic tin;

6 g/l of silver methylsulfonate corresponding to 3.2 g/l of Ag⁺, based on atomic silver;

5.4 g/l of L-cysteine having the empirical formula C₃H₇NO₂S;

350 ml/l of 98% strength methylsulfonic acid having the empirical formula HSO₃CH₃;

50 ml of commercial wetting agent, in this case SolderON SC Primary™ (ShipleyRonal);

5 ml of a further commercial wetting agent, in this case SolderON SC Secondary™ (ShipleyRonal);

water

At an electrolytic bath temperature of 25° C. and a current density of 3 A/dm², well-adherent and matt tin-silver alloy layers having a tin content of 94 wt. % and a silver content of 6 wt. % were obtained.

Example 4

An electrolytic bath having a pH of less than 1 is prepared using the following components:

1.3 g/l of tin methylsulfonate corresponding to 0.5 g/l of Sn²⁺, based on atomic tin;

9.4 g/l of silver methylsulfonate corresponding to 5.0 g/l of Ag⁺, based on atomic silver;

10.1 g/l of L-cysteine having the empirical formula C₃H₇NO₂S;

350 ml/l of 98% strength methylsulfonic acid having the empirical formula HSO₃CH₃;

water

At an electrolytic bath temperature of 25° C. and a current density of 1.6 A/dm², well-adherent and bright tin-silver alloy layers having a tin content of 25 wt. % and a silver content of 75 wt. % were obtained. 

1. An electrolytic bath for electrodepositing silver-tin alloys, which in addition to water as a solvent with a pH of less than 1.5 comprises a water-soluble silver compound, a water-soluble tin compound and an organic complexant, wherein the complexant is an aliphatic complexant and has a sulfide group and an amino group which are linked to different carbon atoms.
 2. The electrolytic bath as claimed in claim 1, wherein the sulfide group and the amino group are linked to adjacent carbon atoms.
 3. The electrolytic bath as claimed in claim 1, wherein the complexant has fewer than seven carbon atoms.
 4. The electrolytic bath as claimed in claim 1, wherein the complexant has a disulfide bridge.
 5. The electrolytic bath as claimed in claim 1, wherein the complexant is a compound from the group consisting of cysteamine, cysteine, cystamine or cystine.
 6. The electrolytic bath as claimed in claim 1, wherein the concentration of the silver ions is 0.1-20 g/l, based on silver.
 7. The electrolytic bath as claimed in claim 1, wherein the concentration of the tin ions is 1-50 g/l, based on tin.
 8. The electrolytic bath as claimed in claim 1, wherein the particle concentration of the complexant is greater than the particle concentration of the silver.
 9. The electrolytic bath as claimed in claim 1, wherein the tin compound is selected from the group of Sn(II) halides, Sn(IV) halides, alkali metal stanates, tin alkanesulfonates or is ammonium stanate, Sn(II) sulfate or SnO, tin citrate or tin oxalate.
 10. The electrolytic bath as claimed in claim 1, wherein the silver compound is silver nitrate, silver sulfate, Ag₂O, a silver halide, a silver alkanesulfonate or a silver diamine complex.
 11. The electrolytic bath as claimed in claim 1, wherein the acid used to adjust the pH is an alkanesulfonic acid.
 12. An electrolytic bath as claimed in claim 1, which comprises brighteners, cross-linking agents and conducting salts in the form of customary additives.
 13. The use of an electrolytic bath as claimed in claim 1 to produce a tin-silver alloy. 